Rolling is a central process due to the place of metal sheets in different industrial sectors (automotive, aeronautics, industrial construction, etc.). Hard chromium has proven an efficient coating for rolling mill rolls, a surface treatment that improves both the durability of the rolls and the productivity (reduction-speed couple) of rolling, especially in the carbon steel and light alloy rolling industries. But the effects on the health of deposition bath operators are known, and in a world where health and the environment are rightly taking on an increasingly crucial place, it is urgent to work to find a replacement that can at least ensure the same level of productivity and durability of rolls.
This is the task that has been tackled in this work, which has brought together rolling specialists, rolling industries and a vacuum deposition company. The shortest way providing an indisputable answer would be to deposit the coating on the rolls of rolling mills and to measure its performance by rolling strips with these rolls, but two major obstacles opposed this: the size of the machine necessary for coating rolling mill rolls and the legitimate reluctance of rolling companies to use a coating directly on the rolls without prior laboratory proof of its performance in this application.
Therefore, as in [19], this work has been focused on measuring properties of coated roll steels relevant for strip rolling performance and comparing with hard chromium among others. Rolling has therefore been analyzed and the phenomena determining the performance of the rolls have been identified -the level and the stability of friction because friction plays a dual role in rolling, being both the driving force of the strips and a source of energy loss and other related harmful phenomena, (excessive heating, profile and flatness defects, chattering...). - adhesion, e.g. seizure or adhesive transfer of the strip metal to the coated roll: transfer by its accumulation can be at the origin of severe strip surface degradation in addition to its negative influence on the friction. - fatigue wear and spalling, because the rolling generates high cyclic subsurface stresses which the coating must resist, knowing that the coatings naturally include a point of weakness inherent to the substrate-coating interface.
PVD/PECVD TiN, CrN, TiBN and a 20% hydrogenated DLC coating were tested and compared to hard chromium and uncoated steel currently used, as all four have displayed high mechanical and tribological performance in other demanding industrial applications.
The two mechanical tests selected address precisely the abovementioned properties. The Amsler test is traditionally devoted to fatigue wear and, in case of coated rollers, spalling ; indeed, major spalling has been shown for the CrN coating [11] and subsurface (Hertz point) fatigue has been detected on the non-coated steel roller [11]. On the contrary, DLC had superior performance in this test compared to the other solutions, showing only mild wear. De Almeida et al. also showed that another, softer DLC (39% H) resisted wear better in a ball-on-flat test than Cr, even when coated on a very rough, EDT-textured roll [19].
The other test, plate-on-ring, has been used for a long time in the steel rolling industry to measure friction and in particular its evolution when seizure, galling and adhesive transfer set in. Its purpose is to determine critical conditions versus lubricant composition or roll surface state (coatings, roughness...). Here, it has proven discriminant, showing in particular early adhesion of high-C steel on TiN ; yet a TiN PVD coating has been advocated by Jimbo and Azushima [9] for law-carbon cold rolling ; clearly, a deeper comparative work would be necessary to understand the difference (roughness, oxidation of the TiN surface...). In this test as well, the deposited DLC displayed stable friction and no or little steel adhesion until the highest loads, outperforming the other coatings in the last part of the tests .
The performance balance of the 6 different conditions is summarized in Table 3. Note that there is still a margin of progress for DLCs in this application, since the lubricants used in the rolling industry and in the present work are optimized for steel rolls (and maybe, secondarily, for Cr coatings), whereas it has been demonstrated that DLC best responded to other types of tribological additives which promote hydrogenation / oxidation / hydroxylation of the DLC surface [14, 15, 18].
Table 3. Coatings performances in different tested phenomena
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Performances in different phenomena
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Severe adhesive wear
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Friction level and stability
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Transfer and surface state evolution
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Fatigue resistance
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Uncoated steel
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Standard
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Standard
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Standard
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Standard
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Hard chromium
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High
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High
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High
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High
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TiN
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Low
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High
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Low
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High
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CrN
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High
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High
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High
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Low
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TiBN
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High
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High
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High
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High
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DLC
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High
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High
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Very high
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High
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As stated above, a difficulty with the development of such PVD or CVD coatings in the rolling industry is the size of the rolls compared with the size of the PE-CVD reactors, contrary to electrodeposition baths as used for Cr. In the course of this research, a large-scale CVD/PVD reactor has been constructed and commissioned, which can coat rolls with a length of ca. 3 m and 1 m in diameter [10]. This is still insufficient for the biggest rolls of plate mills or steel tandem mills, but a number of smaller scale rolling mills, typically for high added value thin strips, can now be treated on this equipment.
De Almeida et al [19] were interested in finishing rolling, which explains why their study focused on the analysis of the stability of friction and the evolution of the surface state due to wear. In this study we are interested in production rolling for this reason we analyze in addition to friction (level and stability) and surface state, the appearance of severe adhesive wear, a phenomenon which limits the workload and consequently the reduction of the strips and the rolling productivity and, due to the cyclic nature of rolling stresses, we also analyzed the fatigue resistance of the coating and we compared all the results with those obtained with the uncoated steel and hard chromium currently used in cold rolling but also with three other PVD coatings.